from __future__ import print_function

'''
Counting Sundays
Problem 19

You are given the following information, but you may prefer to do some research for yourself.

1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one,
Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.

A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?
'''

days_per_month = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

day = 6
month = 1
year = 1901

sundays = 0

while year < 2001:
    day += 7

    if (year % 4 == 0 and not year % 100 == 0) or (year % 400 == 0):
        if day > days_per_month[month - 1] and month != 2:
            month += 1
            day = day - days_per_month[month - 2]
        elif day > 29 and month == 2:
            month += 1
            day = day - 29
    else:
        if day > days_per_month[month - 1]:
            month += 1
            day = day - days_per_month[month - 2]

    if month > 12:
        year += 1
        month = 1

    if year < 2001 and day == 1:
        sundays += 1

print(sundays)
